The present invention relates generally to a holographic viewing device and a computer-generated hologram for the same, and more particularly to holographic spectacles that enable bright images to be viewed in a stable manner and a computer-generated hologram for the same.
U.S. Pat. No. 5,546,198 has proposed holographic spectacles constructed as shown in the perspective view of FIG. 7(a). As shown, two transmission holograms 2 and 3 are fitted in the two-eye sections of a spectacle frame 1. When the spectacles are used to view a scene including such limited extent light sources 4, 5, 6 and 7 as shown in FIG. 7(b), the user would see it as if shown in FIG. 7(c) as an example. In other words, the user would see the pre-selected patterns xe2x80x9cNOELxe2x80x9d 8, 9, 10 and 11 in place of the light sources 4, 5, 6 and 7 in the natural scene of FIG. 6(b). For the transmission holograms 2 and 3 having such characteristics, Fourier transform holograms (Fraunhofer holograms) of the aforesaid pattern xe2x80x9cNOELxe2x80x9d designed as computer-generated holograms are used.
In a computer-aided Fourier transform hologram, a limited rectangular area including a pattern (for instance, xe2x80x9cNOELxe2x80x9d as mentioned above) recorded in that hologram is divided into a matrix array of cells, so that information regarding a pattern portion corresponding to each cell site can be allocated to each cell, so that a pattern comprising a limited number of cells is projected onto a hologram area far away from it upon Fourier transform. As is the case with the pattern recording area, the hologram area, too, is divided into a matrix array of cells to record amplitude information and phase information at each cell site where the pattern to be recorded is subjected to Fourier transform.
Thus, the Fourier transform computer-generated hologram with pre-selected patterns recorded in it comprises a finite number of cells, and so the diffraction efficiency is not always high. Further, the pattern viewed through the above holographic spectacles is not always bright, and higher-order diffraction images are superposed on that pattern. Furthermore, higher-order diffraction images are seen around and adjacent to the main pattern. Thus, this computer-generated hologram has not sufficient easy-to-view properties.
Such a computer-generated hologram is now fabricated by means of photolithography using a mask made by fast Fourier transformation. However, a photolithographic pattern for that mask is very fine, and so it is not easy to fabricate computer-generated holograms capable of reconstructing a given pattern in a stable fashion.
In view of such problems with the prior art, the object of the present invention is to provide a holographic viewing device that has a high diffraction efficiency, enables bright patterns to be viewed in place of light sources in a scene without noticing conjugate images or high-order images if any, and is easy to fabricate with consistent characteristics, and a computer-generated hologram for the same.
According to the present invention, the above object is achieved by the provision of a holographic viewing device comprising a frame and a computer-generated hologram constructed as a transmission Fourier transform hologram and fitted in said frame, characterized in that said computer-generated hologram comprises minuscule cells having pitches xcex4x and xcex4y, with a reconstruction image area defined by a range of spreading of xc2x1 first-order diffracted light of given wavelength from a diffraction grating having grating pitches 2xcex4x and 2xcex4y that are twice as large as said pitches of cells, and an input image pattern, reconstructed at said wavelength in a range of up to ⅔ of said reconstruction image area, is recorded in said computer-generated hologram.
Preferably in this case, said input image pattern, reconstructed in a range of up to xc2xd of said reconstruction image area for said computer-generated hologram, should be recorded in said computer-generated hologram.
Preferably, said computer-generated hologram should comprise a phase hologram.
Preferably, said computer-generated hologram should have a phase distribution multivalued to four or more levels.
The present invention also provides a computer-generated hologram constructed as a transmission Fourier transform hologram for a viewing device, wherein said computer-generated hologram comprises minuscule cells having pitches xcex4x and xcex4y, with a reconstruction image area defined by a range of spreading of xc2x1 first-order diffracted light of given wavelength from a diffraction grating having grating pitches 2xcex4x and 2xcex4y that are twice as large as said pitches of cells, and an input image pattern, reconstructed at said wavelength in a range of up to ⅔ of said reconstruction image area, is recorded in said computer-generated hologram.
Preferably in this case, said input image pattern, reconstructed in a range of up to xc2xd of said reconstruction image area for said computer-generated hologram, should be recorded in said computer-generated hologram.
Preferably, said computer-generated hologram should comprise a phase hologram.
Preferably, said computer-generated hologram should have a phase distribution multivalued to four or more levels.
In the present invention, the computer-generated hologram comprises minuscule cells having pitches xcex4x and xcex4y, with a reconstruction image area defined by a range of spreading of xc2x1 first-order diffracted light of given wavelength from a diffraction grating having grating pitches 2xcex4x and 2xcex4y that are twice as large as said pitches of cells, and an input image pattern, reconstructed at said wavelength in a range of up to ⅔ of said reconstruction image area, is recorded in said computer-generated hologram. It is thus possible to achieve a Fourier transform hologram comprising a computer-generated hologram for a holographic viewing device, which enables bright patterns with less noticeable conjugate or higher-order images to be viewed in place of light sources in the scene that is seen through the viewing device, and is easy to fabricate with consistent characteristics.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts, which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.